1. Field of the Invention
The present invention relates generally to the non-thermal annealing of crystalline materials, and more specifically to the mechanical annealing of crystalline materials.
2. Description of the Background Art
Semiconductor wafers must be annealed following ion implantation to activate dopants as well as to heal damage caused by ion bombardment. The thermal annealing procedures in use now tend to have detrimental side effects. For example, dopant diffusion smears the spatial sharpness of the vertical p-n junctions and degrades the lateral definition of device features. As is well known, implantation can significantly alter the transport properties of both extrinsic impurities and intrinsic defects, often increasing the diffusion coefficients by orders of magnitude. Segregation and gettering of the dopants are also known to occur during thermal annealing. Furthermore, undesirable impurities can be introduced from the surroundings and by diffusion from either the substrate or the top surface.
In some devices, thermal annealing is especially ineffective. An example is devices made from Hg.sub.1-x Cd.sub.x Te which is the leading material for mid-wave and long-wave infrared detectors. Whereas the first Hg.sub.1-x Cd.sub.x Te detectors were photoconductive, the drive toward 2D focal plane arrays (FPAs) with many discrete elements (e.g., 128.times.128) has led to the development of a second generation of pixelated photo-voltaic (PV) devices, which are bump-bonded to silicon CCDs for read-out and on-chip electronic processing. In order to achieve the required spatial patterning of N.times.N individual p-n junctions, many of the leading manufacturers currently employ a fabrication technology based on shallow-junction ion implantation. In the case of p-on-n photodiodes (presently the industry standard for most applications) arsenic or phosphorus ions are implanted into the top layer of the Hg.sub.1-x Cd.sub.x Te, which is initially undoped or lightly doped during epilayer growth.
It is universally found the Hg.sub.1-x Cd.sub.x Te is n-type immediately following bombardment, independent of which dopant species is implanted. The first goal of the anneal is therefore to reduce the n-type background by healing damage and activating the P or As ions. Before annealing only 1 in about 1000 dopant ions occupies a normal lattice site. Ideally, one would prefer that all of the dopants occupy Te sites, where they become single acceptors. However, even following a successful thermal anneal, the activation efficiencies for both p-type and n-type implants in Hg.sub.1-x Cd.sub.x Te tend to remain relatively low, typically about 10%.
A key feature of Hg.sub.1-x Cd.sub.x Te is the exceptional extent to which its properties are governed not only by the presence of extrinsic impurities, but also by stoichiometry. Hg vacancies are by far the most common type of acceptor in unintentionally-doped material. During a thermal anneal, the vacancy concentration can either increase or decrease significantly, depending on whether a Hg overpressure is employed, but it rarely remains fixed. Other native defects, including the majority of those resulting from implantation damage, are known to produce donors. It should be emphasized that thermal annealing strongly influences both the concentrations and spatial distributions of both p-type and n-type stoichiometry-induced defects. Thus even apart from considerations of the impurity activation, thermal history plays a crucial role in governing the detector's electrical properties. It is this extreme sensitivity to stoichiometry that makes it so difficult to maintain fine control over the doping levels and spatial delineations of the p and n junction regions of a Hg.sub.1-x Cd.sub.x Te photodiode. Although spatial redistribution of the implanted ions is much less of a concern when rapid thermal annealing (RTA) is employed, heat-induced changes in the concentration of stoichiometric dopants occur regardless of the time scale of the temperature increase. In particular, while it is often desirable to employ a light doping level in the undamaged n region of the device, type-conversion during the thermal anneal (even by RTA) makes it too difficult to maintain small net doping concentrations using conventional methods.
SIMOX is another example of a wafer that is difficult to anneal thermally. SIMOX is silicon with an insulating oxygen layer one micron below the surface. It is difficult to anneal thermally due to the loss of the thin layer during the thermal cycle. SIMOX is, therefore, an ideal candidate for mechanical annealing.
Mechanical energy may also be used to modify the physical properties of materials other than semiconductors. Examples include strain and stress relief, sintering of refractory materials, altering the magnetic properties of thin film surfaces, and the alteration of flux pinning in high temperature superconductors.